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Bifacial PV Performance Models: Comparison and Field Results
Chris Deline, B. Marion, S. MacAlpine (NREL)
J. Stein (SNL), F. Toor (U. Iowa), S. Ayala (U. Ariz.)
BiFiPV 2017 Workshop – Konstanz Germany
October 26, 2017
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• Project overview
• Rear irradiance models
• Field validation
• Edge effects
• Irradiance nonuniformity
Outline
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3-Yr Bifacial Research Project (2016-2018)
• Module scale o Adjustable rack IV curves (height, tilt,
albedo, and backside shading effects)
o Spatial variability in backside irradiance
o Effects of backside obstructions
• String scale o Fixed tilt rack (tilt, mismatch effects)
o Single axis tracker (investigate potential)
o Two-axis tracker
• System scale o String level monitoring on commercial
systems (validation data)
Collaborative project between Sandia, NREL and University of Iowa (pvpmc.sandia.gov/pv-research/bifacial-pv-project/)
Task 1: Measure Outdoor Bifacial Performance
Stein, J. S., D. Riley, M. Lave, C. Deline, F. Toor and C. Hansen (2017). Outdoor Field Performance of Bifacial PV Modules and Systems. 33rd European PV Solar Energy Conference and Exhibition. Amsterdam, Netherlands. SAND2017-10254
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• Bifacial_Radiance software release github.com/cdeline/bifacial_radiance
• Configuration analysis publication1 o Effect of row spacing, tilt optimization
o Validation of model using Sandia field data
Ray Tracing simulation
1 A. Asgharzadeh et al, “Analysis of the impact of installation parameters and system size on bifacial gain and energy yield of PV systems”, IEEE PVSC 2017 2B. Marion et al., “A Practical Irradiance Model for Bifacial PV Modules”, IEEE PVSC 2017 . https://www.nrel.gov/docs/fy17osti/67847.pdf
Screenshot – Bifacial_Radiance software
View Factor model • BifacialVF software release
github.com/cdeline/bifacialVF
• Method publication2 o Model detail and configuration
o Validation of model using NREL field data
• Integration with SAM software scheduled 2018
View Factor ground reflection geometry
Task 2: Develop Performance Models
3-Yr Bifacial Research Project (2016-2018)
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Radiance CumulativeSky pre-processor
Robinson, Stone “Irradiation modelling made simple: the cumulative sky approach” 2004
Typical ray-tracing approach: use Perez model to generate hourly sky description
o Runtime = hours for annual simulations
CumulativeSky approach: sum annual hourly irradiance into 145 sky patches
o Runtime = seconds for annual simulation.
Single hourly Perez sky (W/m2) Annual cumulative sky conditions (kWh/m2)
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Clearance
Modeling Rear Irradiance – parameters to consider
Tilt
Albedo
Others: Spacing between cells Shade obstructions
Size of array / simulation Location Weather Sky Diffuse Model
Image: http://opsun.com/mounting-solutions/flat-roof/bifacial-pv-racking/
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Bifacial Gain Calculation
* V. Fakhfouri IEC TS 60904-1-2 ED1 (2017)
Our focus today
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PVSyst – 2D “unlimited sheds” bifacial model
o 6.6.4 update increased bifacial response
Solar World “Boost Calculator” – web interface
o Empirical model, not climate sensitive
http://www.solarworld.de/fileadmin/calculator
Industry Bifacial Models for comparison
PVSyst 6.6.4 bifacial interface SolarWorld online calculator
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PVSyst – Bifacial rear irradiance calculation
Front Irradiance Rear Irradiance
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Model intercomparison - height
Low tilt high albedo rooftop application Richmond VA, 1.5m row spacing, 10o tilt, 0.62 albedo
10
15
20
0 0.5 1
Re
ar ir
rad
ian
ce (
%)
Module Ground Clearance (m)
NREL RayTrace
NREL VF PVSyst 6.6.4
SolarWorld
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Model intercomparison – row spacing
Not as good agreement
10
15
20
1 2 3 4
Re
ar ir
rad
ian
ce (
%)
Row to Row pitch (m)
NREL RayTrace
NREL VF
PVSyst 6.6.4
SolarWorld
Low tilt high albedo rooftop application Richmond VA, 0.15 m height, 10o tilt, 0.62 albedo
Field Validation: 3-row mock array Adjustable spacing, tilt, height
Field Validation: 3-row mock array Low ground clearance configuration
Front & rear irradiance sensors
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Mock array configuration - 4 rear, 2 forward facing irradiance
• Tilt= 10deg.
• Slant length = 610mm 1
• Height = 100 - 500mm 0.16 - 0.9
• Row spacing = 900 - 1800mm 1.5 - 3
• Albedo = 0.62
• Compare measured with site modeled conditions
(normalized)
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• 2 months field data
• RayTrace model reflects finite experiment size at high ground clearance.
Mock Array – comparison with NREL models - Height
5%
10%
15%
20%
25%
0 0.2 0.4 0.6 0.8 1
Re
ar ir
rad
ian
ce
Module Ground Clearance (m)
RayTrace model (3 rows)
View Factor model
Measured
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• OK agreement. Additional conditions under test
Mock Array – comparison with models – Row spacing
5%
10%
15%
20%
1 1.5 2 2.5 3
Re
ar ir
rad
ian
ce
Row to Row spacing (m)
RayTrace model
View Factor model Measured
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0%
10%
20%
30%
40%
50%
1 3 5 7 9 11 13 15
Re
ar ir
rad
ian
ce (
%)
Landscape Modules Per Row
Height = 3 m
Height = 0.25 m
Height = 1 m
0%
10%
20%
30%
40%
50%
1 3 5 7 9 11 13 15
Re
ar ir
rad
ian
ce (
%)
Number of Rows
Height = 3 m
Height = 0.25 m
H = 1 m
System Modeling – Edge Effects
Richmond VA, 1.5 m row spacing, 10o tilt, 0.62 alb. 1m landscape module width 20 modules, 3 rows default
# Modules / row
Total # Rows
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0%
10%
20%
30%
40%
50%
1 3 5 7 9 11 13 15
Re
ar ir
rad
ian
ce (
%)
Landscape Modules Per Row
Height = 3 m
Height = 0.25 m
Height = 1 m
0%
10%
20%
30%
40%
50%
1 3 5 7 9 11 13 15
Re
ar ir
rad
ian
ce (
%)
Number of Rows
Height = 3 m
Height = 0.25 m
H = 1 m
System Modeling – Edge Effects
Richmond VA, 1.5 m row spacing, 10o tilt, 0.62 alb. 1m landscape module width 20 modules, 3 rows default
# Modules / row
Total # Rows
* = 1% edge effect
*
*
* = 1% edge effect
* *
*
*
www.nrel.gov
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Thank you!
Chris Deline National Renewable Energy Laboratory
[email protected] github.com/cdeline
This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory
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BACKUP SLIDES
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• Ground divided into n segments in row-to-row direction and shading determined for each
• Irradiance on each ground segment found using view of the sky (configuration factors)
• Rear side irradiance is sum of sky, ground reflected, object reflected components
• Runtime 4 seconds for annual simulation
NREL View Factor Model1
1B. Marion, “A Practical Irradiance Model for Bifacial PV Modules”, IEEE PVSC, 2017.
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Ongoing work: Single-axis tracking
Improvement: the view factor model has been extended to apply to bifacial tracking PV systems. Field validation is underway.